An inkjet printing device is an example of a fluid ejection device that provides drop-on-demand ejection of fluid droplets. In general, inkjet printing devices print images by ejecting ink droplets through a plurality of nozzles onto a print medium, such as a sheet of paper. The nozzles are typically arranged in one or more arrays, such that properly sequenced ejection of ink from the nozzles causes characters or other images to, be printed on the print medium as the printhead and the print medium move relative to each other. In a specific example, a thermal inkjet printhead ejects droplets from a nozzle by passing electrical current through a heating element to generate heat and vaporize a small portion of the fluid within a firing chamber. In another example, a piezoelectric inkjet printhead uses a piezoelectric material actuator to generate pressure pulses that force fluid droplets out of a nozzle.
Although inkjet printing devices provide high print quality at reasonable cost, continued improvement relies on overcoming various challenges that remain in their development. For example, air bubbles are a continuing problem in inkjet printheads. During printing, air from the ink is released and forms bubbles that can migrate from the firing chamber to other locations in the printhead and cause problems such as blocking ink flow, degrading the print quality, causing partly full print cartridges to appear empty, and ink leaks. In addition, pigment-ink vehicle separation (PIVS) remains a problem when using pigment-based inks. Pigment-based inks are preferred in inkjet printing as they tend to be more durable and permanent than dye-based inks. However, during periods of storage or non-use, pigment particles can settle or crash out of the ink vehicle (i.e., PIVS) which can impede or completely block ink flow to the firing chambers and nozzles in the printhead. Other factors such as evaporation of water (for aqueous inks) and solvent (for non-aqueous inks) can also contribute to PIVS and/or increased ink viscosity and viscous plug formation which prevent immediate printing after periods of non-use.